B2 receptor dysfunction in asthmatic subjects may result from impaired catecholamine binding at the B2 receptor sites. This impaired binding may result from altered stereochemistry of the cell membranes associated with major antigenic determinants of the HL-A or MLR loci. This proposes to determine (1) if binding of tritiated isoproterenol to lymphocytes from asthmatics is less than to the lymphocytes of their normal siblings, (2) whether impaired binding is a graded phenomena which correlates with bronchial reactivity as determined by Mecholyl inhalation, and (3) if abnormalities of isoproterenol binding to the lymphocytes or bronchial reactivity to Mecholyl is associated with certain histocompatibility loci detected by mixed lymphocyte cultures. Lymphocyte suspensions (10 to the 6th power cells/ml) will be prepared from lymphocytes collected by Ficoll-Hypaque density gradient centrifugation of 30cc of freshly drawn heparinized blood. Isoproterenol will be measured by scintillation counting of lymphocytes incubated for 1 hour in RMPI-1640 10 to the minus 6th power N and 10 to the minus 9th power N tritiated isoproterenol (SA 1-10 Ci/mM) washed, and filtered through a millipore filter. Histocompatibility of siblings will be assessed by a microtechnique of mixed lymphocyte culture in MEM-S employing incorporation of C14 labelled thymidine (.1 micron Ci/mM) to measure cross stimulation. Bronchial reactivity will be assessed by graded inhalational challenge with Mecholyl (0-10mg/ml, 0-80 inhalations) while sequentially monitoring FEV1 on a digital readout (ACPI #1500) of a rolling seal spirometer (ACPI #220) until there is either a 15 percent fall in FEV1 or 166 cumulative Mecholyl inhalations. Means of isoproterenol binding to lymphocytes will be compared for groups of subjects categorized by Mecholyl inhalation response and for groups of siblings categorized by MLR histocompatibility. Both the Mecholyl reactivity characteristics and the isoproterenol binding characteristics of groups of histocompatible siblings will be compared by establishing four fold tables.